Polyamic acids and esters

ABSTRACT

This invention is concerned with an improved polyimide which makes an excellent electrical insulation for metal conductors. It may be used as the insulating layer around the periphery of the metal conductor or as an adhesive for tape used as the insulating layer. The invention is also concerned with a novel prepolymer which is essentially a polyamide in structure and which is readily curable to the corresponding polyimide. The prepolymer is prepared by reacting a tetraalkyl ester of benzophenone tetracarboxylic acid or its anhydride with 1,3-bis(paminophenoxy)benzene.

United States Patent [191 Bilow 1 POLYAMIC ACIDS AND ESTERS [75]Inventor: Norman Bilow, Los Angeles, Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Sept. 5, 1972 [21] Appl. N0.: 275,676

Related US. Application Data [63] Continuation-impart of Ser. No.211,431, Dec. 23,

1971, abandoned.

[52] 11.8. C1 260/471 R, 161/214, 260/519 [51] Int. Cl. C07C 103/30 [58]Field of Search 260/471 R, 519

[56] References Cited UNITED STATES PATENTS 3726.831 4/1973 Acle et a]260/471 R Primary Examiner-Lorraine A. Weinberger Assistant Examiner-L.A. Thaxton Attorney, Agent, or Firm-W. H. MacAllister; .l. A. Sarjeant[57] ABSTRACT This invention is concerned with an improved polyimidewhich makes an excellent electrical insulation for metal conductors. Itmay be used as the insulating layer around the periphery of the metalconductor or as an adhesive for tape used as the insulating layer. Theinvention is also concerned with a novel prepolymer which is essentiallya polyamide in structure and which is readily curable to thecorresponding polyimide. The prepolymer is prepared by reacting atetraalkyl ester of benzophenone tetracarboxylic acid or its anhydridewith l,3-bis(p-aminophenoxy)benzene.

8 Claims, 2 Drawing Figures POLYAMIC ACIDS AND ESTERS filed Feb. 9,1971; Ser. No. l 13.927, filed Feb. 9, I97 I;

. and Ser. No. l59,025, filed July I, |97l each, now RELATED APPUCATIONSabandoned. Other related art in this field includes U.S.

Pat. No. 3,349,061 and U.S. Pat. No. 3,567,684. This is acontinuation-in-part of my co-pending appli- 5 cation Ser. No. 211,431filed Dec. 23, 1971 now aban- DESCRIPTION OF THE DRAWINGS doned. FIG. 1is a graph comparing the resistance of the insulation provided anelectrical wire by a polyimide tape BACKGROUND OF THE INVENTION I havinga 3 mil overcoat of the polyimide ofthis invent 1O tron as compared withthe same insulated wire without The use of polyimides as insulatinglayers for metallic h overcoah conductors has received extensiveattention, particu-. FIG, 2 i a modifi ti n f FIG 1 i hi h h Overlarly,for those uses where the insulating layer will be. coat i 2 il hi kexposed to high temperatures, e.g., 600F. In addition, there has beenconsiderable effort expended in devell5 BRIEF SUMMARY OF THE INVENTIONoping polyimide precursors which may be directly The present inventionprovides novel polyamides coated on metallic conductors and'then curedto form which can be used to form insulating coatings for elecaprotective insulating layer or which may be used as trical conductors.They are also useful as adhesives or adhesives for bonding insulatinglayers together on overcoatings, particularly for insulating tapes mademetal conductors, particularly where the insulating 20 from heterocyclicpolymers. The novel polyamides layer isa polyimide tape. consistessentially of the following recurring unit:

0 o 0 Ito-t,

i l-O R 11H". as

Typical polyimides now in production are illustrated wherein n is from 1to about 10, preferably 1 to 5, and as follows: R is H or preferablyalkyl having fromone to about five E i ews 4.

*E t t ii i These are used to form insulation tape. 5 carbon atoms.

Other improved polyimide and pyrrone materials, in- The invention isalso concerned with improved insucluding precursors thereof, aredisclosed in co-pending lated metal conductors having a layer, about thepeapplications Ser. No. 851 ,462, filed Aug. 19, 1969; Ser.. riphery ofsaid conductor, of a polyimide consisting es No. 856,237. filed Aug. 8,I969; Ser. No. 113,816, sentially of the following recurring unit:

60 l i ii i O 0' N/\ -\N ad/am, l l n The polyimide is produced bycuring a polyamide of formula I.

The polyimide product of this invention is unique as a coating in thepolyimide art in that it absorbs significantly less water thanconventional polyimides and provides a more flexible wire coating orovercoating for metallic conductors.

GENERAL DESCRIPTION The polyimide precursors (polyamides) of the presentinvention are prepared by condensing substantially equimolecularquantities of a tetraalkyl ester of benzophenone tetracarboxylic acid orthe corresponding anhydride with l,3-bis(p-aminophenoxy) benzene or 1,3-bis(m-aminophenoxy)benzene to form what is basically a polyamidestructure. If excess diamine is used, the polymer will not imidizecompletely and thus will not convert completely to its most heatresistant form. If excess ester or anhydride is used, the polymermolecular weight will be depressed and poorer physical properties couldresult. The reaction may be carried out in a solvent or by melting thetwo reactants and maintaining them in a molten condition for a suitableperiod of time. When the tetra ester is employed, the degree of reactioncan be measured by trapping alcohol which is distilled out of thereaction zone and measuring the volume of the evolved entrapped alcohol.A product produced by the interreaction of 1 mole of the tetra ester andl mole of the diamino compound and condensed to a point where onlyone-third of the alcohol is liberated which theoretically can be formedfrom the four ester groups, will correspond to formula I in which n isbetween i and 2. When n is l in formula I, there will be three estergroups on the benzophenone radical, one amino group'on the terminalbenzene radical and one CONH- bridge linking the benzophenone radicalwith the phenyl ether moiety. At lower temperatures and/or in thepresence of a solution, the reaction tends to be linear involving one ofthe vicinal ester groups with a primary amino group. The reaction iscontrolled so that the resultant product produced from the tetraethylester will have an average molecular weight from approximately 716, whenn is equal to l, to approximately 6,746, when n is equal to 10.

When the reaction involves the benzophenone dianhydride, it does notproduce alcohol. Instead, the dianhydride structure is split to form aCONH bridge between the benzophenone radical and the phenyl ether moietyand the vicinal keto radial is converted to COOH. The molecular weightsof the products will also vary slightly from those produced from thetetra ester because the COOH group is lighter than any of thecorresponding ester groups, and the terminal dianhydride structure islighter than the terminal diester groups. Since an alcohol is notproduced from the dianhydride reaction, an additional method formeasuring the degree of reaction is needed. A commonly used methodinvolves measuring the inherent viscosity of the resultant product. Thisis determined by dissolving 2.4 grams of the polyamide product in 100 ccof dimethylformamide and measuring the viscosity of the solution at 25C.Desirable products are obtained when the inherent viscosity ranges fromapproximately 0.042 to approximately 0.08. This test is equallyapplicable to the products obtained from the reaction involving thetetra ester moiety.

Those skilled in the art can readily determine the reaction conditionsnecessary to achieve a product with a certain inherent viscosity. Withthe benefit of a minimum number of trail runs in light of the specificexamples contained herein, one can quickly establish the temperature andtime of reaction needed with any given benzophenone reaction to obtainrepeatedly the desired viscosity.

Recovery of the polyamide from the reaction vessel will vary slightlydepending upon the mode of reaction. For example, when the product isprepared from a melt of the two reactants, it will solidify on cooling.As a result, it may be poured into a suitable container while still inthe molten condition or allowed to cool in the reaction vessel to form asolid of substantially pure polyamide product. If the reaction has beencarried out in the presence of a solvent, the reaction mixture may bediluted with a liquid in which the solvents remain soluble but in whichthe reaction product is insoluble, thus forcing the reaction product toprecipitate out of solution. Useful liquids for this purpose includemethanol, ethanol, possibly dioxane and propanol. In the alternative,the reaction solvent may be removed under vacuum at temperatures up toabout 100C. Higher temperatures should be avoided to prevent prematureformation of polyimides.

It is not always necessary to remove the solvent, or even to allow thereaction product to solidify. In the case of appropriate reactionsolvents, the resultant solution after reaction may be a useful varnish.The varnish can be coated on wire directly, particularly if the varnishis sufficiently viscous to form a layer of desirable thickness on thewire. If greater viscosity is desired, some of the reaction solvent canbe removed under vacuum. The molten product produced by the meltreaction may also be used in the same manner provided care is taken toavoid premature polyimide formation. Multiple coatings may be used onthe wire itself or on a tape-wrapped wire to build up the thickness ofthe coat. I have found that is is generally advisable to allow thereaction product to dry between coatings.

When the polyamide is used as an adhesive, it can be brushed on or thetape may be dipped into the solution or melt. The coated tape then iswrapped around the periphery of the metallic conductor, and subjected toa B-stage curing step, preferably at a temperature from about 175C toabout 250C for a period from about 1 hour to about 6 hours. Temperaturesabout C lower 50 may be used if the anhydride is used in place of thetetraester. At this temperature the polyamide will flow and form acoherent adhesive between the tape and the wire. Particularly usefuladhesives are obtained when the reaction product is produced from thetetra ester benzophenones and especially the tetraethyl compound; theseproducts allow the tape to bond to itself in an overlapped state in asurprisingly void free manner.

The following examples are set forth as illustrative of my inventions.

EXAMPLE 1 Benzophenonetetracarboxylic acid dianhydride (8.24 grams,0.0255 mole) dissolved in 38 ml of N-methyI-Z-pyrrolidone was placed ina three necked 300 ml flask fitted with a stirrer, a condenser protectedfrom the atmosphere with a dessicant-fllled tube, thermometer andheating mantle. To the flask was added a solution ofl,3-bis(4-aminophenoxy)benzene (10. grams, 0.0342 mole) in 38 ml of N,Ndimjethylacetamide. The solution waskept at 55C fox 16 hours. The totalweight of the lacquer was 89 grams. It contained the equivalent ofpercent solids content.

The lacquer was poured into a large excess of methanol, approximately500 ml, causing the polyamide product to precipitate from solution. Theprecipitate then was separated by filtration on a Buchner funnel, washedwith an additional 100-200 ml of methanol and allowed to dry at roomtemperature.

EXAMPLE 11 Example I was repeated except that the polyamide was notprecipitated from the lacquer. The lacquer was applied directly to awire by drawing the wire through the lacquer a plurality of times untilthe desired coating thickness had been achieved. The wire was allowed todry each time that it was drawn through the lacquer and the finallycoated wire was postcured at 300C for approximately 4 hours.

Ten grams of the solidified product of Example-l may be dissolved in 50ml of chloroform, to form a lacquer having 20% solids. The solidifiedproduct will dissolve readily if it is first pulverized. This lacquercan be used in the manner set forth in this example with equivalentresults. 7

EXAMPLE III 0.10 moles of tetraethyl benzophenone tetracarboxylate and0.10 moles of 1,3-di(4-aminophenoxy)benzene were dissolved in 100 ccdioxane. The solution was placed. in a three-necked, 500 ml flask whichwas fitted with a stirrer, a condenser protected from the atmosphere, athermometer and a heating mantle. The solution temperature was raised to75C and maintained at that temperature for. 10 hours. During thereaction 8 mlof ethyl alcohol was collected in the trap. The reactionsolution was poured into 750 ml of ethanol to precipitate the polyamide,which was separated by filtration, washed and dried. The dried productwas pulverized and then dissolved in chloroform to provide a lacquerhaving percent solids. This lacquer can be used in the same manner asthe lacquer in Example II to coat a wire directly or to coat a polyimidetape which was then used as an insulating layer around the periphery ofa conductor wire. In either event, the product is dried in warm air for2-4 hours, then cured in argon at 6 hours at 288C.

EXAMPLE IV Pieces of a conventional polyimide film (/2 X 1 inch) werecoated with the varnish prepared in Example ll, warm air dried for 2hours and then cured in argon for 6 hours at 288C (550F). The filmsamples, along with two controls, were vacuum dried for 16 hours at 65C,then cooled to room temperature in vacuum and weighed. Next, they wereimmersed in water overnight, wiped dry and reweighed. They were thenredried in vacuum at 65C. Samples coated with the varnish of Example 11picked up 2.6 percent water, which was lost in total when redried. Incontrast, the uncoated, conventional polyimide picked up 4.3 percentwater,

EXAMPLE V Conventional polyimide tapes ,6 X 12 X 0.003 inch) werebrush-coated on one side with the varnish of Example II and after simpleair drying, two-ply laminates were prepared and press-cured under thefollowing conditions:

343C (650F), 150 psi 15 minutes 343C (650F), 150 psi '1 hour Thesesamples showed only marginal adhesion; thus, other samples were driedunder more stringent conditions, namely, at 149C for 30 minutes, 149Cfor 45 minutes and 149C for 60 minutes. These samples subsequently werecured at 16 psi at 200C, 300C and 400C. Although no adhesion wasobserved on samples cured at 200C, adhesion was observed on those curedat 300C to 400C.

Quantitative bond strength measurements thus were warranted andconsequently sheets of'sodium etched conventional polyimide film (3 X 6inches) were brushcoated with the 'prepolymer varnish of Example 11.

These sheets were dried at 65C (140F) in air, then in vacuum at C for 16hours, then they were bonded together at 232C and 100 psi for 2 hours.They were post-cured overnight at 260C. A l X 6 inch strip was cut fromthe laminate and the remaining 2 X 6 inch portion was post-cured again,this time for 16 hours at 288C. The 2 X 6 inch strip was cut in half andone portion was post-cured a third time at 316C for 16 hours. Peel testssubsequently were made at 288C and in all cases the substrate failedrather than the adhesive. This demonstrated its excellent adhesiveproperties in laminate form.

EXAMPLE Vl Experiments were also conducted to demonstrate the use of thepolymer as an overcoat for composite wrapped insulation. A wire A wasprepared by wrapping it with a conventional 1 mil thick polyimide filmknown as Kapton previously coated with an adhesive prepared from thetetraethyl ester of benzonephenonetetracarboxylic acid and 3 ,3 ',4 ,4tetraaminobenzophenone as shown in my copending application, Ser. No.856,237, filed Aug. 18, 1969. A second wire B was prepared using thesame Kapton film and a commercially available polyimide adhesive.

Overcoats of the polyamide of the present invention was applied byrepeated brushings of a 20 percent by weight solution thereof in amixtureof chloroform and tetrahydroturan, air drying for 4hours betweencoats, then oven drying for 16 hours at 60C, followed by 4 hours at 200Cand 4 hours at 290C. Insulation wall thicknesses were built up to thepoint where there was 3 mils of insulation on A and 2 miles insulationon B. The results of insulation resistance measurements are shown in thetable below. In addition, ambient temperature tests were conducted inanionic wetted water and elevated tests in a molten Bi-Sn alloy bath.Graphs of these data are shown in FIGS. 1 and 2 where 0 represents theresistance of the base wire without the overcoat of the presentinvention.

Insulation Resistance versus Temperature of Insulated Wire Overcoatedwith Bis(paminophenoxy)benzene Based Polyimide made as in Example [IInsulation Resistance-30 inch Wires, Ohms TempeFrature Uncoated 3 MilOvercoat Uncoated 2.0 Mil Overcoat Wire A on Wire A Wire B on Wire B 250l2l 7 X10" 3 X l 3 X 4 X 10 500 260 2 X I0 1 X 10 2 X 10 3 X 10 l5 Whatis claimed is: 6. A novel precursor of claim 2 wherein the phenyl l. Apolyimide precursor consisting essentially of the following recurringunit: ether moiety is i? i R0-C o 0 ll 0 n wherein MS from l to about10, and R is H or alkyl hav- 0 0 ing from 1 to about 5 carbon atoms. 1

2. A novel precursor of'claim 1 wherein n is an average number fromabout 1 to about 5.

3. A novel precursor of claim 2 wherein R is H. 4. A novel precursor ofclaim 2 wherein R is ethyl.

5. A novel precursor of claim 2 wherein the phenyl 7. A novel precursorof claim 5 wherein R is ethyl ether moiety is and n is an average numberfrom 1 to about 5.

O O 8. A novel precursor of claim 6 wherein R is ethyl and n is anaverage number from 1 to about 5.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,855,273 Dated December 17, 1974 Inventor) Norman Bilow It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the Specification Column 1, line 13, the comma should be deletedafter "particularly".

Column 3, line 49, "radial" should be "radical" line 60, "2.4" should be"2.5".

Column 5, line I, the printing at the end of the line is faint. Thefinal term of the line should read line 2, the printing at the end ofthe line is faint.

The final term of the line should read "N,N"

line 3, the printing at the end of the line is faint. The final word ofthe line should read "for" line 27, "in the manner set forth in thisexample" should read "in the manner set forth above in this example".

Column 6, line 52, "was" should be "were".

line 59, "miles" should be mils" Column 7, In the Table, the symbol "9"should appear over both "Uncoated Wire A" and "Uncoated Wire B".

In the Claims Claim 6, In the Formula, the left benzene ring should havea bond as indicated below.

0 Signed and Scaled this [SEAL] ntieth D y f January 1976 A ttest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner ofParentsand Trademarks

1. A POLYIMIDE PRECURSOR CONSISTING ESSENTIALLY OF TH FOLLOWINGRECURRING UNIT:
 2. A novel precursor of claim 1 wherein n is an averagenumber from about 1 to about
 5. 3. A novel precursor of claim 2 whereinR is H.
 4. A novel precursor of claim 2 wherein R is ethyl.
 5. A novelprecursor of claim 2 wherein the phenyl ether moiety is
 6. A novelprecursor of claim 2 wherein the phenyl ether moiety is
 7. A novelprecursor of claim 5 wherein R is ethyl and n is an average number from1 to about
 5. 8. A novel precursor of claim 6 wherein R is ethyl and nis an average number from 1 to about 5.